From the prior art according to references U.S. Pat. No. 6,765,335 B1 or U.S. Pat. No. 7,218,031 B2 ultrasonic motors for a rotation drive are for example known, in which the ultrasound actuator is formed as a piezoelectric hollow cylinder, and in which electrodes are disposed on its circumference surfaces and friction elements are disposed on its end surfaces.
In the case of these ultrasonic motors both the polarization vector p as well as the vector of the electric excitation field E are arranged radially and thus perpendicular to the axial direction of the piezoelectric cylinder. As a result the dynamic force Fad generated by the ultrasound actuator in the axial direction and the dynamic force Ftd generated by the ultrasound actuator in the tangential direction are determined by the same piezomodule d31. In limit cases, these forces can be of identical magnitude.
The drive principle of these ultrasonic motors is based on the transmission of the force generated by the ultrasound actuator to the rotor via an effective or friction contact existing between ultrasound actuator and rotor. Based on this drive principle, the friction force Ffk with regard to the friction contact must always be greater than the amplitude of the dynamic force Ftd. If this is not true, there exists an undesired slip between ultrasound actuator and rotor, which results in a deteriorated function or even malfunction of the ultrasonic motor.
The friction force as a result of the friction contact between ultrasound actuator and rotor is determined by the friction coefficient Kf of the contacting materials and the contact pressure force Fa. For modern hard, non-abrasive materials this coefficient is about 0.2-0.3. For ensuring a linear working range of the friction contact, the condition Ffk=Kf×Fa must be maintained. This means that the friction contact of the ultrasonic motor must comprise the friction force Ffk, and that the friction element is pressed to the friction surface of the rotor by a force Fa=Ffk/0.2-0.3. The contact pressure force Fa must therefore be about 3-5 times greater than the amplitude of the force Ftd. The contact pressure force Fa comprises the static contact pressure force Fas and the dynamic contact pressure force Fad, with the force Fas being generated by a spring. For the optimum function of the friction contact, the static contact pressure force Fas must be equal to the amplitude of the dynamic contact pressure force Fad.
In the ultrasonic motors known from the prior art, the actuator develops both the force Fad acting in the axial direction and the force Ftd acting in the tangential direction. This can be explained by the fact that both forces are determined by the same piezomodule d31. Accordingly, in these motors the amplitude of the contact pressure force Fa can only be twice larger than the amplitude of the force Ftd.
Accordingly, the friction force Ffk in friction contact is also limited and together with it the maximum force obtainable by the ultrasonic motor is also limited, so that with the ultrasonic motors known from the prior art it is not possible to generate the nominally obtainable maximum forces achievable with the piezoelectric parameters.
Moreover very high mechanical losses in friction contact (slip) are created with these ultrasonic motors under high loads.
Apart from this, such motors require a comparatively high excitation voltage, whereby the losses in the actuators are increased.
All of these effects together result in heating of the actuators and in a corresponding reduction of the maximum operating temperature. Altogether the ultrasonic motors known from the prior art according to references U.S. Pat. No. 6,765,335 B1 or U.S. Pat. No. 7,218,031 B2 present a comparatively low efficiency.